When zinc ions are treated with aqueous ammonia, what is observed?

• A. White precipitate insoluble in excess.
• B. White precipitate soluble in excess to form a colourless solution.
• C. Red-brown precipitate insoluble in excess.
• D. No reaction.

Answer: (B)

With zinc ions, aqueous ammonia first acts as a base to form zinc hydroxide, which appears as a white gelatinous precipitate. If you add more ammonia, the zinc ions are coordinated by ammonia ligands to give a soluble complex, [Zn(NH3)4]2+. This complex is colorless, so the white precipitate dissolves in excess ammonia and the solution becomes colorless. The other options don’t fit because there is an initial precipitate formed, and it does dissolve in excess ammonia rather than staying insoluble.